Wireless or radio communication between separated electronic devices is widely used. For example, a wireless local area network (WLAN) is a flexible data communication system that may be an extension to, or an alternative for, a wired LAN within a building or campus. A WLAN uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. Accordingly, a WLAN combines data connectivity with user mobility, and, through simplified configurations, also permits a movable LAN.
Over the past several years, WLANs have gained acceptance among a number users including, for example, healthcare, retail, manufacturing, warehousing, and academic areas. These groups have benefited from the productivity gains of using hand-held terminals and notebook computers, for example, to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized and used as a general purpose connectivity alternative for an even broader range of users. In addition, a WLAN provides installation flexibility and permits a computer network to be used in situations where wireline technology is not practical.
In a typical WLAN, an access point provided by a transceiver, that is, a combination transmitter and receiver, connects to the wired network from a fixed location. Accordingly, the access transceiver receives, buffers, and transmits data between the WLAN and the wired network. A single access transceiver can support a small group of collocated users within a range of less than about one hundred to several hundred feet. The end users connect to the WLAN through transceivers which are typically implemented as PC cards in a notebook computer, or ISA or PCI cards for desktop computers. Of course the transceiver may be integrated with any device, such as a hand-held computer.
The assignee of the present invention has developed and manufactured a set of integrated circuits for a WLAN under the mark PRISM 1 which is compatible with the proposed IEEE 802.11 standard. The PRISM 1 chip set is further described in Harris Corporation Application Note entitled “Harris PRISM Chip Set”, No. AN9614, March 1996; and also in a publication entitled “PRISM 2.4 GHz Chip Set”, file no. 4063.4, October 1996.
The PRISM 1 chip set provides all the functions necessary for full or half duplex, direct sequence spread spectrum, packet communications at the 2.4 to 2.5 GHz ISM radio band. In particular, the HSP3824 baseband processor manufactured by Harris Corporation employs quadrature or bi-phase phase shift keying (QPSK or BPSK) modulation schemes. While the PRISM 1 chip set is operable at 2 Mbit/s for BPSK and 4 Mbit/s for QPSK, these data rates may not be sufficient for higher data rate applications.
Spread spectrum communications have been used for various applications, such as cellular telephone communications, to provide robustness to jamming, good interference and multi-path rejection, and inherently secure communications from eavesdroppers, as described, for example, in U.S. Pat. No. 5,515,396 to Dalekotzin. The patent discloses a code division multiple access (CDMA) cellular communication system using four Walsh spreading codes to allow transmission of a higher information rate without a substantial duplication of transmitter hardware. U.S. Pat. No. 5,535,239 to Padovani et al., U.S. Pat. No. 5,416,797 to Gilhousen et al., U.S. Pat. No. 5,309,474 to Gilhousen et al., and U.S. Pat. No. 5,103,459 to Gilhousen et al. also disclose a CDMA spread spectrum cellular telephone communications system using Walsh function spreading codes.
Unfortunately, the conventional Walsh function spreading codes may create undesirable signal components for some applications. Moreover, a WLAN application, for example, may require a change between BPSK and QPSK during operation, that is, on-the-fly. Spreading codes may be difficult to use in such an application where an on-the-fly change is required.